Thermoelectric materials having large figure of merit (ZT) are desirable for power conversion and refrigeration technologies. Thin-film materials have pushed the limits and extended the application of thermoelectrics to niche markets where the high cost of thin-film devices is not as important as their efficiency. If bulk materials with low-cost processing and high figure of merit can be developed, their application to mainstream markets may be realized. (See Yang, J. and T. Caillat, Thermoelectric materials for space and automotive power generation, MRS Bulletin, 2006, 31 (3): p. 224-229.) Current state of the art bulk materials typically have ZT maxima of approximately 1 at operating temperatures dependent on the band gap of the material. These include Bi2Te3 (ZT=0.9 at 400K) and PbTe (ZT=0.8 at 600K) and for high temperature applications SiGe alloys (ZT=0.9 at 1200K). (See Tritt, T. M., M. A. Subramanian, and Editors, Harvesting Energy Through Thermoelectrics: Power Generation and Cooling, [In: MRS Bull.; 2006, 31 (3)], 2006, 113 pp.)
The figure of merit is defined as ZT=S2σT/κ where S is the Seebeck coefficient, σ is the electrical conductivity, and κ the thermal conductivity. (See Rowe, D. M., CRC handbook of thermoelectrics, 1995, Boca Raton, Fla.: CRC Press, 70 1; Rowe, D. M., Thermoelectrics handbook: macro to nano, 2006, Boca Raton: CRC/Taylor & Francis, 1 v. (various pagings).) The thermal conductivity is the combination of heat carried by phonons or lattice vibrations (κlat) and electrical carriers (κelec). Many techniques have been developed for reducing the thermal conductivity in order to increase ZT such as solid solution alloying, nanostructuring (see Quarez, E., et al., Nanostructuring, Compositional Fluctuations, and Atomic Ordering in the Thermoelectric Materials AgPbmSbTe2+m, The Myth of Solid Solutions, Journal of the American Chemical Society, 2005, 127 (25): p. 9177-9190; Poudeu, P. F. P., et al., Nanostructures versus solid solutions: Low lattice thermal conductivity and enhanced thermoelectric figure of merit in Pb9.6Sb0.2Te10-xSex bulk materials, Journal of the American Chemical Society, 2006, 128 (44): p. 14347-14355; Poudeu, P. F. R., et al., High thermoelectric figure of merit and nanostructuring in bulk p-type Na1−xPbmSbyTem+2, Angewandte Chemie-International Edition, 2006, 45 (23): p. 3835-3839; Androulakis, J., et al., Nanostructuring and high thermoelectric efficiency in p-type Ag(Pb1−ySny)(m)SbTe2+m, Advanced Materials, 2006, 18 (9): p. 1170-+; Sootsman, J. R., et al., Strong reduction of thermal conductivity in nanostructured PbTe prepared by matrix encapsulation, Chemistry of Materials, 2006, 18 (21): p. 4993-4995), and investigation of new structures. (See Kanatzidis, M. G., Structural Evolution and Phase Homologies for \“Design\” and Prediction of Solid-State Compounds, Accounts of Chemical Research, 2005, 38 (4): p. 359-368; Nolas, G. S., J. Poon, and M. Kanatzidis, Recent developments in bulk thermoelectric materials, MRS Bulletin, 2006, 31 (3): p. 199-205.) The use of solid solution alloying has been used to increase phonon scattering at point defects within solids and is well understood. And although solid solution scattering is useful, the majority of phonons at the higher temperatures, where materials for power generation are most needed, are more effectively scattered by features on the nanoscale. (See Kim, W., et al., Thermal conductivity reduction and thermoelectric figure of merit increase by embedding nanoparticles in crystalline semiconductors, Physical Review Letters, 2006, 96 (4), Li, D., et al., Thermal Transport in Nanostructured Solid-State Cooling Devices, Journal of Heat Transfer, 2005, 127 (1): p. 108-114; Majumdar, A., Materials science: Thermoelectricity in semiconductor nanostructures, Science (Washington, DC, United States), 2004, 303 (5659): p. 777-778.) Materials engineered on the nanoscale have exhibited ZT values as high as 3 in thin-film materials (see Harman, T. C., et al., Nanostructured thermoelectric materials, Journal of Electronic Materials, 2005, 34 (5): p. L19-L22) and higher than 2 in bulk. (See Hsu, K. F., et al., Cubic AgPbmSbTe2+m: Bulk Thermoelectric Materials with High Figure of Merit, Science (Washington, DC, United States), 2004, 303 (5659): p. 818-821.) The increases in ZT reflect the low thermal conductivity brought about by phonon scattering at the interfaces of nanoscale features and the “matrix” of the semiconductor with values of the lattice thermal conductivity approaching 0.35 W/mK. Drawbacks in thin-film materials such as their cost and processing difficulties have prompted the exploration of new methods to prepare bulk nanostructured materials. Such materials have been realized through the use of processes such as matrix encapsulation (see Sootsman, J. R., et al., Strong reduction of thermal conductivity in nanostructured PbTe prepared by matrix encapsulation, Chemistry of Materials, 2006, 18 (21): p. 4993-4995), spinodal decomposition (see Androulakis, J., et al., Spinodal Decomposition and Nucleation and Growth as a Means to Bulk Nanostructured Thermoelectrics: Enhanced Performance in Pb1-xSnxTe—PbS, Journal of the American Chemical Society: p. ACS ASAP), and nucleation and growth. (See Heremans, J. P., C. M. Thrush, and D. T. Morelli, Thermopower enhancement in PbTe with pb precipitates, Journal of Applied Physics, 2005, 98 (6).)
A report by Snyder and coworkers describes the decomposition of a metastable Pb2Sb6Te11 into PbTe and Sb2Te3 through a eutectoid reaction. (See Heremans, J. P., C. M. Thrush, and D. T. Morelli, Thermopower enhancement in PbTe with Pb precipitates, Journal of Applied Physics, 2005, 98 (6); Ikeda, T., et al., Solidification processing of alloys in the pseudo-binary PbTe—Sb2Te3 system, Acta Materialia, 2007, 55 (4): p. 1227-1239.) This method produced a lamellar structure of alternating phases with layer period ranging from approximately 200-950 nm depending on annealing temperature and times. Others have explored eutectic type phase relationships for thermoelectric applications although they typically were composed of a semiconductor (III-V and IV-VI compounds) and a metal (Te, Sb, and others). (See Isakov, G. I., Phonon Scattering, Thermoelectric Power, and Thermal Conductivity Control in a Semiconductor-Metal Eutectic Composition, Semiconductors, 2005, 39 (7): p. 738-741; Isakov, G. I., Control of electric and thermal properties of composites with whiskers, Journal of Engineering Physics and Thermophysics (Translation of Inzhenerno-Fizicheskii Zhurnal), 2004, 77 (5): p. 1062-1068; Park, C.-G., B.-G. Min, and D.-H. Lee, Thermoelectric properties of unidirectionally solidified Bi2Te3-PbBi4Te7 eutectic alloy, Han'guk Chaelyo Hakhoechi, 1995, 5 (2): p. 251-8; Dement'ev, I. V. and V. V. Leonov, The effect of temperature on the thermoelectric properties of the eutectic alloys of the systems AIIIBIV-germanium, Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy, 1988, 24 (1): p. 24-7; Leonov, V. V. and Z. K. Gantimurova, Thermoelectric properties of a eutectic alloy of indium arsenide with germanium, Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy, 1987, 23 (11): p. 1915-17; Leonov, V. V. and Y. E. Spektor, Thermoelectric properties of the germanium-gallium arsenide eutectic alloy, Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy, 1980, 16 (8): p. 1358-60.) The use of oriented eutectics has also been shown in thermoelectric composites where the Seebeck coefficient, electrical and thermal conductivity can be tuned by the angle with respect to growth direction. (See Isakov, G. I., Phonon Scattering, Thermoelectric Power, and Thermal Conductivity Control in a Semiconductor-Metal Eutectic Composition, Semiconductors, 2005, 39 (7): p. 738-741; Isakov, G. I., Control of electric and thermal properties of composites with whiskers, Journal of Engineering Physics and Thermophysics (Translation of Inzhenerno-Fizicheskii Zhurnal), 2004, 77 (5): p. 1062-1068; Leonov, V. V., Properties of eutectic alloys of the systems AIIIBV-germanium (silicon) prepared by directed crystallization, Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy, 1985, 21 (2): p. 320-1; Liebmann, W. K. and E. A. Miller, Preparation, phase—boundary energies, and thermoelectric properties of InSb—Sb eutectic alloys with ordered microstructures, Journal of Applied Physics, 1963, 34 (9): p. 2653-9.)